The accurate measurement of temperatures between 1100° C. and 1700° C. is important to the safe, efficient and economical operation of many industries such as electrical power production, processing and refining of chemicals, the fabrication of steel and other metals, and production of glass and ceramic materials. Accurate temperature measurement over time can also be critical to the operation of industrial machinery such as jet engines, nuclear reactors, gasification units, incinerators, and gas turbines. In such temperature environments, thermocouples are the most widely used industrial temperature sensors because they are rugged, affordable and accurate—at least initially.
Unfortunately, after installation all commercial thermocouples are unstable in this temperature range and prone to decalibration or “drift,” providing increasingly unreliable and unpredictable readings as they age. As operating temperatures and thermal cycles increase, the performance of these thermocouples decreases. Together, these factors often result in costly redundant instrument clusters, sensor failures, downtime and potential accidents due to undetected overheating. For temperatures above 1100° C. in radiation environments, such as in high-temperature nuclear test reactors, conventional thermocouples are incapable of stable and accurate operation.
The thermocouple of the present invention overcomes the two most critical thermocouple issues plaguing high-temperature operations; signal drift and sensor longevity. The first problem with all conventional thermocouple sensors is that they are subject to decalibration. The uncertainties surrounding decalibration are difficult to quantify, but elevated temperatures and longer operating times inevitably result in increasingly unreliable measurements. Standard thermocouples drift appreciably after a few hundred hours of use, making accurate temperature measurement and high-temperature process control difficult without frequent sensor change out. For high-temperature nuclear applications there are even greater limitations. Currently there are no high-temperature thermocouples capable of withstanding neutron flux in nuclear fission reactors or, potentially one day, fusion reactors. The key to minimizing drift lies in selecting thermocouple materials with properties that do not interact with each other or appreciably change during use.
A second problem is that prolonged heating, contamination, and thermal cycling increases brittleness and fragility and shortens thermocouple life. Most metals, including those used in thermocouples, become brittle with high-temperature exposure; as a result, they can fail due to mechanical stresses induced by vibrations, expansion, and contraction. Heat from welding to form standard thermocouple junctions also can lead to mechanical failure from embrittlement. Compatibility of component metals at high temperature and improved joining methods are essential to improved thermocouple durability.
It is an object of the invention to provide a high-temperature thermocouple capable of operating in hostile environments for a long period of time without significant signal degradation.
It is another object of the invention to provide a method of fabricating a high-temperature thermocouple capable of operating in hostile environments for a long period of time without significant signal degradation.
It is still a further object of the invention to provide a thermocouple capable of operating in a temperature range of 1100° C. to 1700° C. in a radiation environment.
Additional object, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and practice of the invention.